Apicomplexa are responsible for a number of important human diseases including malaria, toxoplasmosis, cryptosporidiosis and cyclosporidiosis. Management of these diseases rests heavily on chemotherapy but anti-parasitic drug treatment faces multiple challenges. These include poor overall potency, restriction to certain life-cycle stages, unwanted side effects, and rapidly emerging multiple drug resistance. A constant stream of new drugs and potential drug targets is required to stay abreast of the threat posed by these pathogens. One of the most promising sources of such parasite specific targets is the apicomplexan plastid or apicoplast. The apicoplast is unique to the parasite and its function is essential to parasite survival. This organelle is a holdover from a free-living photosynthetic past. The structure and biology of the apicoplast is remarkably complex as it is derived from the endosymbiotic marriage of two eukaryotes: a red alga and an auxotrophic protist. The goal of this application is to unravel the complexity of this biology in mechanistic detail and to identify future targets for intervention. Using Toxoplasma as a model organism we will conduct genetic, cell biological and biochemical approaches to characterize the function of two pathways that unfold in the outer compartments of the organelle and that we hypothesize are essential to the organelle and the parasites. We will complement this focused approach with a broader effort to define a comprehensive set of plastid proteins to continue to feed a pipeline of hypothesis-driven mechanistic experiments with strong candidate genes.